The High Frequency (HF) band is a unique medium for radio communications that is distinguished from most radio communication mediums by its point to point long distance and wide area capabilities using relatively simple modulation techniques, self contained transceivers and usable without repeaters. The communication distances of HF radio can be hundreds to thousands of kilometers making the medium suitable for vehicle based mobile transceivers located many kilometers from civilization that need to communicate between mobile transceivers or to base stations in less remote locations. The antennae of HF radio communication systems are long compared to most other radio communication systems which mean that mobile and base station platforms are possible but are not viable for handheld transceivers.
However, it is not always convenient to have to operate the HF transceiver from the mobile, especially, when the user works away from the vehicle.
In different radio frequency bands, for example, the Ultra High Frequency (UHF) range and Very High Frequency (VHF) range it is known to have both vehicle-mounted and hand-held transceivers that allow a user remote access to respective communication base station systems and that also may allow mobile-to-mobile or handheld-to-handheld communication between users. In general, the distance over which that is achievable for UHF and VHF transceivers is very small compared with HF vehicle based transceiver communications.
The mode of communication of HF, UHF and VHF systems can vary greatly. Some examples of those modes are:                Single Frequency Simplex (where each user shares a single frequency and takes turns speaking and listening);        Twin Frequency Simplex (where each user takes turns speaking and listening but transmit and receive on two different frequencies that allows the communication system to include a remotely located repeater so as to provide communications over a greater area than can be achieved transceiver to transceiver); for convenience reference herein to simplex systems will include both single and twin frequency simplex operation unless otherwise defined; and        Twin Frequency Duplex (where each user can speak and listen at the same time and which also allows for the use of a repeater).        
Very generally, single frequency simplex mode is used in HF communications; twin frequency simplex is used in land based mobile and handheld communications; and twin frequency duplex is used in cellular communications systems providing a more telephone like experience to the user. Cellular systems generally use radio frequency bands in the upper reaches of the UHF band.
Even with the use of UHF and VHF handheld transceivers, reliable communication distances are restricted to handheld to handheld maximum communication ranges when a repeater is not in range of the handheld. Often though, the user's vehicle based VHF or UHF transceiver is in range and with the greater range achievable with a vehicle based transceiver in the same or another band, it makes sense to provide a connection between the handheld and the vehicle based transceiver. This connection can be achieved a number of ways, each depending on the frequency band of either the handheld or the vehicle transceiver. It is generally easier in simplex systems where the handheld is in one or other of the VHF or UHF bands and the vehicle based transceiver is in the other.
The type of connection described immediately above is part of a cross-band system.
A simple example of a vehicle-mounted cross-band system is depicted in FIG. 1 (prior art) which shows a UHF handheld 10 being used remote of a vehicle 12 which has both a UHF transceiver 14 and VHF transceiver 16 that allows the user to communicate from their handheld via the vehicle to and from a VHF repeater 18 remote from the vehicle. Also depicted is another user 19 communicating in their turn via the VHF repeater 18 to the first mentioned UHF handheld 10.
In the example above, the communications mode is Twin Frequency Simplex and only one user can talk at a time via the remote repeater.
However, in a Single Frequency Simplex system such as one using the HF band, if an HF remote base station is used to communicate over distances to a vehicle mounted HF receiver but the HF receiver receives both the HF transmitted signal and an interference signal, such as a strong Amplitude Modulated commercial radio station, there is a likelihood that a strong enough interfering signal will swamp the vehicle-mounted HF receiver and the signal from the base station will be lost in the noise or made intelligible.
However, furthermore such an occurrence will also adversely affect the operation of a cross-band system designed to pass on the received HF signal to the user of a handheld transceiver, because the unwanted received signal will cause the connected VHF or UHF transmitter to transmit to the handheld radio, for as long as the received signal includes that noise and/or an unintelligible voice signal. Yet further the calling party using the HF link will not be aware of this problem.
Thus use of a single frequency simplex system excludes a single frequency simplex remote handheld (operating in the VHF or UHF bands) doing anything other than receiving unwanted transmitted noise or otherwise intelligible voice signal because the handheld does not have the ability to access the cross-band system to advise the originator of the signal that they can not be heard. This is the case because the respective VHF or UHF receiver at the mobile is blocked from being received while the respective VHF or UHF transmitter in the vehicle is retransmitting the interfering signal received on the HF receiver.
Furthermore the HF receiver is not controllable by the HF remote base station since the HF receiver is swamped or adversely affected by the interfering signal.
For this type of problem, the invention described herein eliminates, reduces or at least provides an alternative operation for a simplex cross-band system.